An example of a prior art accelerometer design with high performance potential is described in U.S. Pat. No. 3,702,073. The accelerometer described therein is comprised of three primary components, namely, a proof mass assembly and upper and lower stators or magnetic circuits between which the proof mass assembly is supported. The proof mass assembly includes a movable reed that is suspended via flexure elements to an outer annular support ring. The reed and support ring are commonly provided as a unitary structure composed of fuzed quartz.
Both upper and lower surfaces of the reed include capacitor plates and force restoring coils. Each force restoring coil is positioned on the reed such that the central axis of the coil coincides with a line that extends through the center of the reed and that is normal to the top and bottom surfaces of the reed. This line is coincident with the sensitive axis of the accelerometer. A plurality of mounting pads are formed at spaced apart positions around the upper and lower surfaces of the annular support ring. These mounting pads mate with inwardly facing surfaces of the upper and lower stators when the accelerometer is assembled.
Each stator is generally cylindrical, and has a bore provided in its inwardly facing surface. Contained within the bore is a permanent magnet. The bore and permanent magnet are configured such that an associated one of the force restoring coils of the proof mass assembly fits within the bore, with the permanent magnet being positioned within the cylindrical core of the force restoring coil. Current flowing through the coil therefore produces a magnetic field that interacts with the permanent magnet to produce a force on the reed. Also provided on the inwardly facing surfaces of the stators are capacitor plates configured to form capacitors with the upper and lower capacitor plates on the reed. Thus movement of the reed with respect to the upper and lower stators results in a differential capacitance change.
In operation, the accelerometer is affixed to an object whose acceleration is to be measured. Acceleration of the object along the sensitive axis results in pendulous, rotational displacement of the reed and coils with respect to the support ring and the stators. The resulting differential capacitance change caused by this displacement is sensed by a suitable feedback circuit. The feedback circuit produces a current that, when applied to the force restoring coils, tends to return the reed to its neutral position. The magnitude of the current required to maintain the reed in its neutral position is directly related to the acceleration along the sensitive axis.
An important characteristic of an accelerometer of the type described above is its immunity to errors due to thermal stress. Thermal stress results from the fact that different parts of the accelerometer are composed of materials that have different coefficients of thermal expansion. For example if the reed is composed of fused quartz, and if the coils are mounted on metal coil forms that in turn are mounted on the reed, temperature change will result in stress at the reed/coil form interfaces that can cause reed deformation and result in errors in the accelerometer output. Deformations are canceled to a large extent by the symmetry of the opposing coil, but small errors may still result.